1. Field of the Invention
This invention relates to a method of and a system for reading out radiation image information stored on a stimulable phosphor sheet in which the stimulable phosphor sheet is exposed to stimulating rays, thereby causing it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation and the light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal representing the radiation image information, and more particularly to an improvement for preventing saturation of a photodetector having a photoelectric surface.
2. Description of the Related Art
When certain kinds of phosphors are exposed to radiation such as X-rays, xcex1-rays, xcex2-rays, xcex3-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as xe2x80x9ca stimulable phosphorxe2x80x9d. It has been known to use stimulable phosphors in radiation image recording and reproducing systems (sometimes referred to as xe2x80x9ccomputed radiographyxe2x80x9d). Specifically, a radiation image of an object, such as a human body, is recorded on a stimulable phosphor sheet (a recording medium provided with a layer of the stimulable phosphor). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is used for reproducing the radiation image of the object as a visible image on a recording medium such as a photosensitive material or a display such as a CRT. See Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, 56(1981)-11397 and the like.
The radiation image recording and reproducing system is practically advantageous in that as compared with conventional radiographies using silver halide film, an image can be recorded over an extremely wide radiation exposure range.
When reading out radiation image information from the stimulable phosphor sheet in the radiation image recording and reproducing system, for instance, a light beam such as a laser beam is caused to two-dimensionally scan the stimulable phosphor sheet storing thereon a radiation image, and the light emitted from the stimulable phosphor sheet upon stimulation thereof is transmitted to a photodetector through an optical guide having a light inlet end face extending along the main scanning line. The photodetector photoelectrically detects in time the light emitted from the stimulable phosphor sheet upon stimulation thereof and an image signal made up of image signal components for respective picture elements is obtained.
The photodetectors generally employed in such systems include those utilizing an internal photoelectric effect, e.g., a phototransistor, a photodiode and the like, and those having a photoelectric surface and utilizing a photoemissive effect on the photoelectric surface, e.g., a photomultiplier. When those having a photoelectric surface such as a photomultiplier are employed in the above system, the following provision is generally made in order to improve sensitivity of the photodetector.
That is, the photodetector of this type is generally provided with an optical guide for collecting the light, emitted from the stimulable phosphor sheet upon stimulation thereof, to the photoelectric surface. The provision involves increasing light collecting efficiency to the photoelectric surface by improving the light collecting performance of the optical guide and/or employment of a photoelectric surface made of a material such as bialkali, e.g., Sb-K-Cs, which is high in quantum efficiency.
However, increasing the sensitivity of a photodetector with a photoelectric surface gives rise to another problem that a saturation phenomenon, that the sensitivity of the photodetector deteriorates for a while after detection of a large amount of light, is apt to occur at the photoelectric surface and when a visible image is reproduced on the basis of an image signal obtained from a saturated photodetector, a ghost image appears and the image quality deteriorates.
The system may be employed in general industrial field as well as a medical field. In the medical field, the object is the human body and since the irradiation dose does not greatly vary depending on the part whose radiation image is to be taken, the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof does not greatly fluctuate.
To the contrast, in the general industrial field, where the system is used in non-destructive inspection of products such as a cast iron block, an iron block and the like, various kinds of materials can be the object.
Accordingly, in order to obtain radiation image information suitable for the respective kinds of materials, the irradiation dose varies depending on the kind of the object over a very wide range (two to three figures in terms of dose ratio).
Thus in the radiation image recording and reproducing system for use in the general industrial field, the aforesaid saturation phenomenon is apt to occur, and accordingly, there has been a demand for a radiation image information read-out system in which the saturation phenomenon at the photodetector is suppressed with the sensitivity of the photodetector kept high.
The intensity of light emitted from the stimulable phosphor sheet upon stimulation thereof rapidly increases from initiation of exposure to the stimulating light and is maximized in a short time (e.g., in several ns) and then is gradually weakened, with the stimulable phosphor sheet keeping emitting light for a time unique to the phosphors on the stimulable phosphor sheet after termination of exposure to the stimulating light. The light emitted from the stimulable phosphor sheet after termination of exposure to the stimulating light is generally referred to as xe2x80x9cafterglowxe2x80x9d. Accordingly when the stimulable phosphor sheet is scanned by the stimulating light and the light emitted from the stimulable phosphor sheet is photoelectrically read out in time series, the afterglow component of picture elements precedingly exposed to the stimulating light is read in addition to the light emitted from a given picture element upon stimulation thereof as the radiation image information component for the picture element, which results in incomplete separation of image signal components for the picture elements and deterioration in sharpness of the reproduced image. Accordingly, when the stimulable phosphors on the stimulable phosphor sheet exhibit long afterglow, the sharpness of the image deteriorates to an unacceptable level.
Such a phenomenon occurs substantially in proportion to the irradiation dose (radiation energy) of the stimulating light per unit area of the stimulable phosphor sheet. Accordingly, for example, when a picture element where the amount of light emitted upon stimulation is relatively small exists just behind a picture element where the amount of light emitted upon stimulation is relatively large, the afterglow component from the large emission picture element is superposed on the light emitted from the small emission picture element upon stimulation thereof, and the amount of light read out as that emitted from the small emission picture element is increased by the amount of the afterglow. This reduces the difference between the image signal components for the large emission picture element and the small emission picture element as compared with the actual difference therebetween, and accordingly the reproduced image deteriorates in contrast, i.e., the S/N ratio of the image signal deteriorates.
In order to overcome this problem, there has been proposed a method in which interference between image signal components for the respective picture elements due to response properties (e.g., attenuation properties) of the light emitted from the picture elements upon stimulation thereof is electrically corrected by adding, to each of the image signal components in time series obtained by scanning the stimulable phosphor sheet with the stimulating light, a differential value of the image signal component. (See Japanese Patent Publication No. 2(1990)-15154.)
However this approach is disadvantageous in the following points. First this approach involves a large amount of calculation since interference between image signal components for the respective picture elements is corrected by calculation taking into account the response properties (e.g., attenuation properties) of the light emitted from the picture elements upon stimulation thereof. Second though being able to avoid deterioration in sharpness and/or contrast of the radiation image, the approach cannot overcome the problem of deterioration in S/N ratio.
In view of the foregoing observations and description, a first object of the present invention is to provide a radiation image information read-out system and method in which the saturation phenomenon at the photodetector is suppressed, thereby preventing appearance of a ghost image, with the sensitivity of the photodetector kept high.
A second object of the present invention is to provide a radiation image information read-out system and method in which said problems caused by the afterglow inclusive of deterioration in S/N ratio can be overcome in a simple manner and the radiation image information can be accurately read out at a high speed even if the scanning speed is increased.
The first object of the present invention can be accomplished by a radiation image information read-out method and a radiation image information read-out system in which the absolute value of the amount of light impinging upon the photoelectric surface of the photodetector is suppressed by causing the light emitted from the stimulable phosphor sheet upon stimulation thereof to pass through a variable transmittance medium whose transmittance is varied according to the amount of light emitted from the stimulable phosphor sheet before impinging upon the photoelectric surface or by changing the amount of stimulating light according to the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof.
That is, in accordance with a first aspect of the present invention, there is provided a radiation image information read-out method for obtaining an image signal representing radiation image information on an object stored on a stimulable phosphor sheet by scanning the stimulable phosphor sheet with a stimulating light beam and photoelectrically detecting light emitted from the stimulable phosphor sheet upon stimulation thereof by a photodetector having a photoelectric surface, wherein the improvement comprises the steps of
inserting a variable transmittance medium whose transmittance to the light emitted from the stimulable phosphor sheet upon stimulation thereof is variable continuously or stepwise into the optical path of the light between the stimulable phosphor sheet and the photoelectric surface of the photodetector, and
changing the transmittance of the variable transmittance medium according to the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof so that the photoelectric surface is not saturated by an excessive amount of light impinging thereupon.
The stimulating light beam may be visible light, a laser beam or the like.
As the variable transmittance medium, an electrochromic element whose transmittance to light varies according to the direction of current, a NCAP type liquid crystal element or the like can be employed as those whose transmittance can be electrically changed. Further a ND filter system in which a plurality of optical elements which are different in transmittance are mechanically selectively inserted into said optical path can also be employed.
As a material for electrochromic element, amorphous WO3 (colorless to blue), IrO2 (colorless to blue), viologen (colorless to blue), anthraquinone (colorless to red) or the like may be used, and may be selected according to the color of light emitted from the stimulable phosphor sheet.
The NCAP (Nematic Curvilinear Aligned Phase) type liquid crystal element is of encapsulated nematic liquid crystals. Generally the liquid crystal has a rod-like molecule exhibiting electrooptic anisotropy. The molecules are apt to orient along an oriented film, and in a normal state where no electric field is applied to the liquid crystal element, the molecules orient inward of the capsules and incident light is scattered at the surface and inside of the liquid crystals depending on the refractive properties of the crystals, whereby the liquid crystal element becomes opaque. When an electric field is applied to the liquid crystal element, the liquid crystals which are positive in dielectric anisotropy orient in the direction perpendicular to the surface of the electrodes. When the liquid crystals are of the same refractive index as the polymer which is outside the capsules and in which the liquid crystals are dispersed, light travels straight without being scattered and accordingly the liquid crystal element becomes transparent.
The variable transmittance medium may be disposed at the light inlet end face of the optical guide or at the connection between the optical guide and the photodetector when the optical guide is integrated with the photodetector. Though not necessary, it is preferred that the optical guide be integrated with the photodetector from the viewpoint of simplicity of handling. In the case of a ND filter system, the filter must be moved to adjust the transmittance and accordingly it is not preferred that the ND filter system is integrated with the photodetector or the like to such an extent that the ND filter cannot make a relative movement.
The degree by which the transmittance of the variable transmittance medium is to be changed according to the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof may be empirically determined and may be tabulated with respect to the amount of light so that the degree by which the transmittance of the variable transmittance medium is to be changed can be known by referring to the table.
The amount of light emitted from the stimulable phosphor sheet upon stimulation thereof may be determined by actually exposing a part of stimulable phosphor sheet, which is limited not to affect reproduction of the radiation image, to the stimulating light and detecting the amount of light emitted from the part or may be determined by estimation based on the irradiation dose to which the stimulable phosphor sheet was exposed to the radiation upon taking the radiation image and the energy of the stimulating light. such estimation may be input from the outside. In the case of the radiation image recording and reproducing system for industrial use, the kinds of objects are larger than in the case of that for medical use. Accordingly, the radiographing menu such as the material and the thickness of the object, the radiographing direction and the like is closely related to the irradiation dose and on the basis of this fact, the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof may be determined according to the radiographing menu.
Also in the case where the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof is determined according to the radiographing menu, the degree by which the transmittance of the variable transmittance medium is to be changed may be empirically determined and may be tabulated in relation to the radiographing menu so that the degree by which the transmittance of the variable transmittance medium is to be changed can be known by referring to the table on the basis of the menu.
The above description may also be applied to the followings inventions.
In accordance with a second aspect of the present invention, there is provided a radiation image information read-out system for carrying out the method in accordance with the first aspect of the present invention. That is, in accordance with the second aspect of the present invention, there is provided a radiation image information read-out system for obtaining an image signal representing radiation image information on an object stored on a stimulable phosphor sheet by scanning the stimulable phosphor sheet with a stimulating light beam and photoelectrically detecting light emitted from the stimulable phosphor sheet upon stimulation thereof by a photodetector having a photoelectric surface, wherein the improvement comprises
a variable transmittance medium which is variable continuously or stepwise in transmittance to the light emitted from the stimulable phosphor sheet upon stimulation thereof and is inserted into the optical path of the light between the stimulable phosphor sheet and the photoelectric surface of the photodetector, and
a transmittance changing means which changes the transmittance of the variable transmittance medium according to the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof so that the photoelectric surface is not saturated by an excessive amount of light impinging thereupon.
In accordance with a third aspect of the present invention, there is provided a radiation image information read-out method for obtaining an image signal representing radiation image information on an object stored on a stimulable phosphor sheet by scanning the stimulable phosphor sheet with a stimulating light beam and photoelectrically detecting light emitted from the stimulable phosphor sheet upon stimulation thereof by a photodetector having a photoelectric surface, wherein the improvement comprises the step of
changing the amount of the stimulating light impinging upon the stimulable phosphor sheet continuously or stepwise according to the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof so that the photoelectric surface is not saturated by an excessive amount of light impinging thereupon.
The amount of the stimulating light impinging upon the stimulable phosphor sheet may be changed by directly controlling the source of the stimulating light to change the amount of stimulating light emitted from the source, or by inserting a variable transmittance medium whose transmittance to the stimulating light is variable continuously or stepwise into the optical path of the stimulating light from the stimulating light source to the stimulable phosphor sheet and changing the transmittance of the variable transmittance medium with the amount of stimulating light emitted from the source unchanged. The variable transmittance medium may be an electrochromic element, a NCAP type liquid crystal element, a ND filter system or the like. When the amount of the stimulating light impinging upon the stimulable phosphor sheet is changed, the levels of the signal representing detection of the leading end of the sheet (in the sub-scanning direction) and the signal representing detection of the scanning starting point (in the main scanning direction) are changed. Accordingly it is preferred that the gains of such signals be automatically adjusted.
The above description may also be applied to the followings inventions.
In accordance with a fourth aspect of the present invention, there is provided a radiation image information read-out system for obtaining an image signal representing radiation image information on an object stored on a stimulable phosphor sheet by scanning the stimulable phosphor sheet with a stimulating light beam and photoelectrically detecting light emitted from the stimulable phosphor sheet upon stimulation thereof by a photodetector having a photoelectric surface, wherein the improvement comprises
a means for changing the amount of the stimulating light impinging upon the stimulable phosphor sheet continuously or stepwise according to the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof so that the photoelectric surface is not saturated by an excessive amount of light impinging thereupon.
In the radiation image information read-out method and system of the first and second aspects of the present invention, the level of the amount of light impinging upon the photoelectric surface of the photodetector is suppressed not to saturate the photoelectric surface by changing the transmittance of the variable transmittance medium disposed in the optical path of the light emitted from the stimulable phosphor sheet upon stimulation thereof between the stimulable phosphor sheet and the photoelectric surface according to an estimated or measured amount of the light emitted from the stimulable phosphor sheet upon stimulation thereof.
For example, when the amount of light is large, the transmittance of the variable transmittance medium is reduced so that a smaller part of the light emitted from the stimulable phosphor sheet can reach the photoelectric surface, whereby generation of image signal components which can produce a ghost image due to saturation of the photoelectric surface can be suppressed.
On the other hand, when the amount of light is small, the transmittance of the variable transmittance medium is kept high so that an image signal can be obtained at a high sensitivity and deterioration in S/N ratio can be suppressed. In this case, since the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof is originally small, there is no fear that the photoelectric surface is saturated.
In the radiation image information read-out method and system of the third and fourth aspects of the present invention, the level of the amount of light impinging upon the photoelectric surface of the photodetector is suppressed not to saturate the photoelectric surface by changing the amount of stimulating light impinging upon the stimulable phosphor sheet according to an estimated or measured amount of the light emitted from the stimulable phosphor sheet upon stimulation thereof.
For example, when the amount of light expected to be emitted from the stimulable phosphor sheet upon stimulation thereof is large, the amount of stimulating light impinging upon the stimulable phosphor sheet is reduced so that a smaller amount of light is emitted from the stimulable phosphor sheet, whereby the level of the amount of light impinging upon the photoelectric surface of the photodetector is suppressed and generation of image signal components which can produce a ghost image due to saturation of the photoelectric surface can be suppressed.
On the other hand, when the amount of light expected to be emitted from the stimulable phosphor sheet upon stimulation thereof is small, the amount of stimulating light is not reduced (if possible, may be increased, for instance, by controlling the stimulating light source) and accordingly, the amount of light emitted from the stimulable phosphor sheet is not reduced, whereby an image signal can be obtained at a high sensitivity and deterioration in S/N ratio can be suppressed. In this case, since the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof is originally small, there is no fear that the photoelectric surface is saturated.
In the radiation image information read-out methods and systems of first to fourth aspects of the present invention, when the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof is estimated on the basis of the radiographing menu, necessity of actually detecting the amount of light is eliminated and algorithm for the system can be simplified.
In accordance with a fifth aspect of the present invention, there is provided a radiation image information read-out method for obtaining an image signal representing radiation image information on an object stored on a stimulable phosphor sheet by exposing the stimulable phosphor sheet to stimulating light and photoelectrically detecting light emitted from the stimulable phosphor sheet upon stimulation thereof by a photodetector, wherein the improvement comprises the step of
setting the sensitivity of the photodetector so that the amount of light emitted from a blank portion on the stimulable phosphor sheet bearing thereon no radiation image information of the object becomes larger than that corresponding to the upper limit of the operable range of the photodetector, and
controlling the irradiation energy of the stimulating light so that the level of a signal component obtained by the photodetector by reading the light emitted from the blank portion is minimized in the range higher than the upper limit of the read-out signal level range corresponding to the operable range of the photodetector.
The xe2x80x9cblank portionxe2x80x9d on the stimulable phosphor sheet bearing thereon no radiation image information of the object is a portion which, when the radiation image was recorded, was directly exposed to the radiation without passing through the object. Further xe2x80x9cthe operable range of the photodetectorxe2x80x9d is a range of the amount of light in which the photodetector can correctly convert the amount of light into an electric signal component.
The expression xe2x80x9ccontrolling the irradiation energy of the stimulating lightxe2x80x9d means to control the effective energy of the stimulating light to which the stimulable phosphor sheet is exposed per unit area thereof. The irradiation energy of the stimulating light can be controlled, for instance, by attenuating the stimulating light from the stimulating light source such as a laser by an acoustooptic modulator or the like provided on the optical path of the stimulating light, or by, in the case where the stimulating light source is a semiconductor laser, controlling the input voltage to the semiconductor laser, or by increasing the scanning speed of the stimulating light beam.
In the radiation image information read-out method of the fifth aspect, it is preferred that xe2x80x9cpreliminary read-outxe2x80x9d be effected prior to xe2x80x9cfinal read-outxe2x80x9d and the sensitivity of the photodetector be set on the basis of the image information obtained by the preliminary read-out.
As disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 58(1983)-67241, 58(1983)-67243 and 58(1983)-83937, the xe2x80x9cpreliminary read-outxe2x80x9d is a well-known technique in which brief of the radiation image information stored on a stimulable phosphor sheet is read out prior to the xe2x80x9cfinal read-outxe2x80x9d by exposing the stimulable phosphor sheet to stimulating light having a lower level than that used in the final read-out and reading out light emitted from the stimulable phosphor sheet upon stimulation by the lower level stimulating light.
A plurality of levels of the sensitivity of the photodetector may be registered in advance in relation to different radiographing menus and the sensitivity of the photodetector may be set to the sensitivity level corresponding to the radiographing menu for the radiation image information to be read out.
The xe2x80x9cradiographing menuxe2x80x9d means the part of the object, the method of radiographing and the like and includes, for instance, chest radiographing, head radiographing, angiography and the like.
In accordance with a sixth aspect of the present invention, there is provided a radiation image information read-out system for obtaining an image signal representing radiation image information on an object stored on a stimulable phosphor sheet comprising a stimulating light projecting means which exposes the stimulable phosphor sheet to stimulating light thereby causing the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon and a photodetector which detects the light emitted from the stimulable phosphor sheet upon stimulation thereof, wherein the improvement comprises
a sensitivity setting means which sets the sensitivity of the photodetector so that the amount of light emitted from a blank portion on the stimulable phosphor sheet bearing thereon no radiation image information of the object becomes larger than that corresponding to the upper limit of the operable range of the photodetector, and
a stimulating energy control means which controls the irradiation energy of the stimulating light so that the level of a signal component obtained by the photodetector by reading the light emitted from the blank portion is minimized in the range higher than the upper limit of the read-out signal level range corresponding to the operable range of the photodetector.
It is preferred that the radiation image information read-out system of the sixth aspect be provided with a preliminary read-out means which effects preliminary read-out prior to final read-out of the radiation image information and the sensitivity setting means sets the sensitivity of the photodetector on the basis of the image information obtained by the preliminary read-out.
Further it is preferred that the radiation image information read-out system of the sixth aspect be provided with a means for registering a plurality of levels of the sensitivity of the photodetector in relation to different radiographing menus and the sensitivity setting means sets the sensitivity of the photodetector to the sensitivity level corresponding to the radiographing menu for the radiation image information to be read out.
In the radiation image information read-out method and system of the fifth and sixth embodiments of the present invention, since the sensitivity of the photodetector is set in the aforesaid manner and the irradiation energy of the stimulating light is controlled in the aforesaid manner, the afterglow component of the light emitted from the blank portion is reduced with the light emitted from the portion other than the blank portion kept at the normal level. Since the afterglow component itself is reduced, noise due to the afterglow component can be reduced and the S/N ratio of the read-out signal can be improved. Further since the light emitted from the portion other than the blank portion is kept at the normal level, the image information for the object necessary for diagnosis can be obtained at a proper quality.
When the preliminary read-out is effected prior to the final read-out and the sensitivity of the photodetector is set on the basis of the image information obtained by the preliminary read-out, setting of read-out condition is facilitated. Further when the sensitivity of the photodetector is set on the basis of the radiographing menu, setting of read-out condition is also facilitated.